aapt: Speed up the style pruning

Prior to this commit it removed each style one by one which
causes the vector to repeatedly be shrunk and reallocated and
copied (aka either quadratic or NlogN in the number of items
removed).

This commit simply makes it remove them all at once to make it
linear instead.

Change-Id: I541d151675ab19f37d9de1e7a323104d0d3b3c63