Allow building plain armeabi binaries for android

Building with "make OS=android APP_ABI=armeabi" will
produce arm binaries that will conform to the armeabi
ABI - not using any features outside of armv5te by
default (but still optionally using the NEON functions at
runtime if detected - even though such devices should rather
use the default armeabi-v7a build).
Use the iMultipleThreadIdc field from SEncParamExt
Use SliceModeEnum as data type for the slice mode fields

This makes the use of the field clearer and safer by allowing
the compiler check that users actually assign proper enum
values.
Support building for any other android architecture

This allows e.g. building for mips, as a plain C++ build with
no assembly.
Make the processing lib use mutexes from WelsThreadLib from the common library

This requires always building the WelsMutex* functions,
even if MT_ENABLED isn't set.
Don't try to set thread scope and scheduling policy on android

These APIs aren't implemented on android.
Mark pointers as const where possible in vaacalc
Fix a typo, "heigth" -> "height"
Fix a typo in arm assembly, LORD -> LOAD
   use global trace in encoder
Merge pull request #462 from mstorsjo/fix-typos

Fix two typos in variable and macro names
Merge pull request #452 from mstorsjo/use-slice-mode-enum

Use SliceModeEnum as data type for the slice mode fields
Merge pull request #445 from mstorsjo/use-thread-param

Use the iMultipleThreadIdc field from SEncParamExt
Merge pull request #460 from mstorsjo/add-const

Mark pointers as const where possible in vaacalc
keep the unrelated changed line
add memalign unit test
specific slice mode in encoder test
Merge pull request #454 from mstorsjo/android-any-arch

Support building for any other android architecture
Merge pull request #465 from ruil2/encoder_trace

   use global trace in encoder
reviewed at https://rbcommons.com/s/OpenH264/r/176/
Merge pull request #467 from sijchen/add_more_slice_mode_test

Specific slice mode in encoder test