Split predictive back bitmask flags into a few boolean flags.

This is such that they can be controlled from Flag Flipper UI.

Boolean flags introduced:
persist.wm.debug.predictive_back - Overall switch for the new back infra
persist.wm.debug.predictive_back_anim - Animation switch
persist.wm.debug.predictive_back_always_enforce - Switch to ignore per
app opt-in flag and always enforce new behavior
persist.wm.debug.predictive_back_screenshot - Screenshot switch. Not
added to Flipper yet.

Test: Flip the flags in Flipper and verify behavior is as expected.

Change-Id: Icff098035119fb83f393e20c6ac590be2f22c845