sf: Add a NULL check in getDisplayConfigs

Validate the display binder by adding a NULL check in getDisplayConfigs.
This will prevent a false match if the caller queries the display
configs for an inactive display (whose binder is NULL by default).

Without this change we might end up attempting to index the display
config array, which is unpopulated for inactive displays, and this will
result in a crash. (See getDisplayInfo in SurfaceComposerClient.cpp for
an example of this scenario)

Change-Id: I1a12f43b7c375b9c01998dadd5b658275c733fb2
sf: Add RGBA_4444 and RGBA_5551 format in getFormatStr function

Add RGBA_4444 and RGBA_5551 format in getFormatStr function to log
string name instead of format number in dumpsys Surfaceflinger log.

Change-Id: Ib8423f27ce6bc0fd1b99e7610a870b39ff6441c0
sf: Rename default app buffer count property

We are setting default app buffer count property from HAL,
and there we can't go with read only based property.
Hence renaming it.

Change-Id: I9fd4bdb50aca35647402c59b24d78ad5f878aab5