Fix Integer Overflow in MPEG4Writer

When size is calculated in estimateMoovBoxSize, the max file size is
first multiplied by 6 and then divided by 1000. The multiplication
operation could result in an Integer Overflow. This change reorders the
multiplication and division to avoid the overflow.

Test: atest StagefrightRecorderTest
Bug: 383383170
Change-Id: I145620bda6930fd2b86a51857b3ea6dcfdf9480a