Move all debug flags in a single place.

This change also adds a new memory usage flag. When turned on, the
following is printed after every frame:

D/OpenGLRenderer( 3723): Current memory usage / total memory usage (bytes):
D/OpenGLRenderer( 3723):   TextureCache          3766680 / 20971520
D/OpenGLRenderer( 3723):   LayerCache            3538944 /  8388608
D/OpenGLRenderer( 3723):   GradientCache          135168 /   524288
D/OpenGLRenderer( 3723):   PathCache               41180 /  4194304
D/OpenGLRenderer( 3723):   TextDropShadowCache         0 /  2097152
D/OpenGLRenderer( 3723):   FontRenderer 0         262144 /   262144
D/OpenGLRenderer( 3723):   FontRenderer 1         262144 /   262144
D/OpenGLRenderer( 3723):   FontRenderer 2         262144 /   262144
D/OpenGLRenderer( 3723): Other:
D/OpenGLRenderer( 3723):   FboCache                    2 /       12
D/OpenGLRenderer( 3723):   PatchCache                 31 /      512
D/OpenGLRenderer( 3723): Total memory usage:
D/OpenGLRenderer( 3723):   8268404 bytes, 7.89 MB

This should help tracking possibe memory issues.

Change-Id: I83f483ca1d2dbef904829bce368e33fe5503e8d6