Camera2: Fix camera application memory leak

This change fix potential memory leak of camera application
when swith between camera and panorama:

 1. free allocated frames in jni/Mosaic
 2. use application context instead of acitivity context
 3. clear disappearing children of ViewGroup explicitly
 4. remove IdleHandler during camera exit

CRs-fixed: 622518
Change-Id: I2583709489ab2cf1fc20865fe7d2a8f56f2763c9