Add key triggers for safe mode during boot.

To disable Xposed during zygote startup:
- Make sure one of the VOLUME keys is held down before app_process starts
- 2 short vibrations will occur when the detection starts (zygote starting)
- press and release POWER or HOME twice, without releasing the VOLUME key that is held down
- release the VOLUME key
- 2 short vibrations will signal that the combo was detected and the disabler file will be created

To enable Xposed, the same steps can be executed but with three presses of POWER/HOME instead of two.
Three vibrations will occur if this was triggered.

If VOLUME was pressed but no valid combination occurs within 10 seconds, a single vibration will
signal that an invalid combination was used.
Remove key trigger to re-enable Xposed
New logic for safemode detection, based on a single key being pressed multiple times.

Upon startup, one of the physical keys must be pressed during the first 2 seconds (or already
held down upfront).
After that, the same key must be pressed again within 5 seconds, until 5 total presses are reached.

Two short vibrations are issued when the initial key is detected, and an additional short vibration
for every subsequent key press.
If the trigger is successful, a long vibration is issued.
Don't trigger safemode if more than one key is pressed initially
Safemode code revision and documentation
Added "safemode_disable" flag file to prevent disabling Xposed with the safemode key sequence during Zygote startup.
If the file exists, safemode detection will be entirely disabled.
Beautifying
Merge branch 'safemode'
v49: Safemode